Ca2+ signaling in the transformation of promastigotes to axenic amastigotes of Leishmania donovani

Three acidic phospholipases A₂ from Indian cobra (Naja naja naja) venom inhibited platelet aggregation in platelet rich plasma induced separately by ADP, collagen and epinephrine with different potencies. The order of inhibition was epinephrine > collagen > ADP. They did not inhibit platelet aggregation induced by arachidonic acid (10 M). The inhibition was dependent on concentration of the protein and the time of incubation of the phospholipases A₂ with platelet rich plasma. Parabromophenacyl bromide modified PLA₂ enzymes lost their enzymatic activity as well as platelet aggregation inhibition activity suggesting the involvement of catalytic function in platelet aggregation inhibitory activity.     

β-Glutamyl Peptides, 3-(2-Furoyl)-alanine and Ascorbalamic Acid in Roots of Rumex obtusifolius L. (Ezo-no-Gishigishi)

We found a potent platelet aggregation inhibitor in the culture broth of Streptomyces strain M-193. The inhibitor was purified as white crystals and investigated; it was identified as staurosporine. It strongly inhibited the platelet aggregation induced by collagen or ADP with IC₅₀ values of 3.4 μm, and 11.6 μm, respectively. The inhibitor had no effect on thrombin-induced platelet aggregation or membrane stabilization against heat-induced hemolysis.     

Antithrombotic effects of peroxynitrite: Inhibition and reversal of aggregation in human platelets

The inhibition of platelet aggregation by peroxynitrite, a reactive oxygen species derived from the interaction of nitric oxide (NO) and superoxide, was examined in platelet-rich plasma. In this report, we have used a preparation of peroxynitrite that was free of H₂0₂ and MnO₂. As such, peroxynitrite dose-dependently (50–200 μA) inhibited aggregation of human platelets stimulated by ADP (5 μM), collagen (0.5 μg), thrombin (0.5 UlmL) and U46619 (1 PM). In addition, peroxynitrite reversed platelet aggregation induced by collagen, ADP, and thrombin. Peroxynitrite, preincubated with platelet-poor plasma or albumin (7%) for 30 min, did not alter the inhibition of platelet aggregation. This suggested that the inhibitory action of peroxynitrite may be due to nitrosylation of proteins, which by themselves possess activity, rather than conversion to NO or NO donors. Furthermore, we show that peroxynitrite increased the cGMP level only at 200 μM concentrations, further suggesting that the action of peroxynitrite was not completely due to its conversion to NO or NO donors.     

An inhibitor selective for collagen-stimulated platelet aggregation from the salivary glands of hard tick Haemaphysalis longicornis and its mechanism of action

Soluble materials of salivary glands fromHaemaphysalis longicornis were found to inhibit collagen, ADP, and thrombin-stimulated platelet aggregation. One inhibitory component was purified to salivary gland homogeneity by a combination of gel filtration, ion-exchange, and C₈ reverse phase HPLC. The purified activity, named longicomin, is a protein of molecular weight 16 000 on SDS-PAGE under both reduced and nonreduced conditions. Collagen-mediated aggegation of platelets in plasma and of washed platelets (IC₅₀ was approximately 60 nmol/L) was inhibited with the same efficacy. No inhibition of aggregation stimulated by other effectors, including ADP, arachidonic acid, thrombin, ristocetin, calcium ionophore A23187, thromboxane A2 mimetic U46619 and 12-O-phorbol-13-myristate acetate, was observed. Longiconin had no effect on platelet adhension to collagen. Not only platelet aggregation but also release reaction, and increase of intracellar ca²⁺ level of platelets in response to collagen were completely eliminated by longicomin. Increasing amounts of collagen are able to overcome the inhibition of aggregation by longicomin, indicating that longicomin probably shares with collagen a common receptor. In addition, collagen fibers did not emit fluorescence after incubation with isothocyanate-conjugated longicornin, indicating that longicomin did not bind directly to collagen fibers. The identification and isolation of longicomin demonstrates the existence of a new type of platelet inhibitor that should be useful to better undentand the mechanism of collagen stimulation of platelets. Project supported by the National Natural Science Foundation of China (Grant No. 39170591).     

Pyruvate metabolism of the hyperthermophilic archaebacterium Pyrococcus furiosus

The hyperthermophilic anaerobe Pyrococcus furiosus was found to grow on pyruvate as energy and carbon source. Growth was dependent on yeast extract (0.1%). The organism grew with doublings times of about 1 h up to cell densities of 1–2×10⁸ cells/ml. During growth 0.6–0.8 mol acetate and 1.2–1.5 mol CO₂ and 0.8 mol H₂ were formed per mol of pyruvate consumed. The molar growth yield was 10–11 g cells(dry weight)/mol pyruvate. Cell suspensions catalyzed the conversion of 1 mol of pyruvate to 0.6–0.8 mol acetate, 1.2–1.5 mol CO₂, 1.2 mol H₂ and 0.03 mol acetoin. After fermentation of [3-¹⁴C]pyruvate the specific radioactivities of pyruvate, CO₂ and acetate were equal to 1:0.01:1. Cellfree extracts contained the following enzymatic activities: pyruvate: ferredoxin (methyl viologen) oxidoreductase (0.2 U mg^-1, T=60°C, with Clostridium pasteurianum ferredoxin as electron acceptor; 1.4 U mg^-1 at 90°C, with methyl viologen as electron acceptor); acetyl-CoA synthetase (ADP forming) [acetyl-CoA+ADP+Piacetate+ATP+CoA] (0.34 U mg^-1, T=90°C), and hydrogen: methyl viologen oxidoreductase (1.75 U mg^-1). Phosphate acetyl-transferase activity, acetate kinase activity, and carbon monoxide:methyl viologen oxidoreductase activity could not be detected. These findings indicate that the archaebacterium P. furiosus ferments pyruvate to acetate, CO₂ and H₂ involving only three enzymes, a pyruvate:ferredoxin oxidoreductase, a hydrogenase and an acetyl-CoA synthetase (ADP forming).Non-standard abbreviations DTE dithioerythritol - MV methyl viologen - MOPS morpholinopropane sulfonic acid - Tricine N-tris(hydroxymethyl)-methylglycine Part of the work was performed at the Laboratorium für Mikrobiologie, Fachbereich Biologie, Philipps-Universität, Karlvon-Frisch-Strasse, W-3550 Marburg/Lahn, Federal Republic of Germany     

An inhibitor selective for collagen-stimulated platelet aggregation from the salivary glands of hard tickHaemaphysalis longicornis and its mechanism of action

Soluble materials of salivary glands fromHaemaphysalis longicornis were found to inhibit collagen, ADP, and thrombin-stimulated platelet aggregation. One inhibitory component was purified to salivary gland homogeneity by a combination of gel filtration, ion-exchange, and C₈ reverse phase HPLC. The purified activity, named longicomin, is a protein of molecular weight 16 000 on SDS-PAGE under both reduced and nonreduced conditions. Collagen-mediated aggegation of platelets in plasma and of washed platelets (IC₅₀ was approximately 60 nmol/L) was inhibited with the same efficacy. No inhibition of aggregation stimulated by other effectors, including ADP, arachidonic acid, thrombin, ristocetin, calcium ionophore A23187, thromboxane A2 mimetic U46619 and 12-O-phorbol-13-myristate acetate, was observed. Longiconin had no effect on platelet adhension to collagen. Not only platelet aggregation but also release reaction, and increase of intracellar ca²⁺ level of platelets in response to collagen were completely eliminated by longicomin. Increasing amounts of collagen are able to overcome the inhibition of aggregation by longicomin, indicating that longicomin probably shares with collagen a common receptor. In addition, collagen fibers did not emit fluorescence after incubation with isothocyanate-conjugated longicornin, indicating that longicomin did not bind directly to collagen fibers. The identification and isolation of longicomin demonstrates the existence of a new type of platelet inhibitor that should be useful to better undentand the mechanism of collagen stimulation of platelets.     

Effect of TFC-612, a 7-thia prostaglandin E1 derivative, on a peripheral arterial occlusive disease model in rats

TFC-612, methyl 6-({(1R,2S,3R)-3-hydroxy-2-{(1E,3S,5R)-3-hydroxy-5-methyl-1-nonenyl}-5-oxocyclopentyl}-thio]-hexanoate, inhibited the progression of the lesion in a lauric acid-induced peripheral arterial occlusive model at 1.0 mg/kg p.o. or 1.0 μg/rat/h s.c. in rats. Aspirin (32 mg/kg, p.o.), an anti-platelet drug, did not suppress the lesion growth. On the other hand, ketanserin (10 mg/kg, p.o.), a 5-HT₂ antagonist, also inhibited the progression of the lesion. In vitro, TFC-612 inhibited rat platelet aggregation induced by collagen and ADP with IC₅₀ values of 5.4 ng/mL and 9.5 ng/mL, respectively. Aspirin also inhibited collagen-induced aggregation with an IC₅₀ value of 6.3 μg/mL, but not ADP-induced aggregation at 180 μg/mL. Ketanserin had no effect on either aggregation at 40 μg/mL. In ex vivo experiments, aspirin inhibited platelet aggregation induced by collagen at 10 and 32 mg/kg in rats. However, TFC-612 showed significant inhibition only at 10 mglkg. TFC-612 and ketanserin increased dermal blood flow in the rat paw at 1.0 μg/kg i.v. and 100 μg/kg Lv., respectively. Aspirin had no effect on blood flow at 3.2 mg/kg i.v. These results suggest that the improvement of microcirculation, in addition to anti-platelet action by TFC-612, contributes to its inhibitory effect in a peripheral arterial occlusive model in rats.     

Inhibition of human platelet aggregation and membrane lipid peroxidation by food spice, saffron

The inhibitory activity of saffron extract was studied on human platelets. Platelet aggregation and lipid peroxidation were evaluated with platelet rich plasma (PRP) and platelet membranes respectively obtained from blood of healthy human volunteers. Human platelets were subjected to stimulation with a variety of agonists like ADP (61 μM), epinephrine (76 μM), collagen (11 μg/ml), calcium ionophore A 23187 (6 μM) and ristocetin (1.25 μg/ml) in the presence and absence of saffron extract with IC₅₀ being 0.66, 0.35, 0.86 and 0.59 mg respectively and no inhibition with ristocetin. The inhibitory effect was dose dependent with concentrations varying between 0.16 to 0.80 mg and time dependent at IC₅₀. A significant decrease was observed in malondialdehyde (MDA) formed, one of the end products of arachidonic acid metabolism and of serotonin released from dense granules of platelets at respective IC₅₀. Lipid peroxidation in platelet membranes induced by iron-ascorbic acid system was inhibited by saffron extract significantly with IC₅₀ of 0.33 mg. Hence, it may be said that aqueous extract of saffron may have component(s), which protect platelets from aggregation and lipid peroxidation. (Mol Cell Biochem 278: 59–63, 2005)     

Novel synthetic RGD analogs incorporating salicylic acid derivatives show antiplatelet activity in vitro

Continuing our investigational efforts for more active GpIIb/IIIa inhibitors we have synthesized four novel RGD (Arg-Gly-Asp) analogs incorporating salicylic acid derivatives at their N-terminal amino group using the solid phase synthesis.The synthesized compounds 5-Cl-2-HO-C₆H₃-CO-Arg-Gly-Asp(OBzl)-NH₂and 5-Br-2-HO-C₆H₃-CO-Arg-Gly-Asp(OBzl)-NH₂ were tested for their inhibitory activity on human platelet aggregationin vitro and found to be potent inhibitors of the platelet aggregation with 75 and 67% inhibitory activity respectively. In order to confirm our results flow cytometry with monoclonal antibodies against GpIb, GpIIb/IIIa, GpIIIa and GMP140receptors was used.     

Studies on structure activity relationship of some dihydroxy-4-methylcoumarin antioxidants based on their interaction with Fe(III) and ADP

Three dihydroxy-4-methylcoumarin (DHMC) derivatives, namely 7,8-DHMC, 6,7-DHMC and 5,7-DHMC alone and complexed with Fe (III) and ADP have been tested for their antioxidative potential. Chemical speciation studies and formation constants reveal the formation of strong DHMC–Fe–ADP (1:1:1) ternary complex. In vitro studies were done for their antioxidative property by scavenging the superoxide radicals (O ₂ ^– ) generated by xanthine + xanthine oxidase (XO) reaction. The IC₅₀ values for 7,8-DHMC, 6,7-DHMC and 5,7-DHMC and their ternary complexes with Fe (III)–ADP worked out to be 34.0 M, 62.0 M, 8.80 mM and 10.5, 11.5 and 148.5 M, respectively. The results indicate that O ₂ ^– scavenging potential of all the three DHMCs increased significantly after forming the ternary complex with Fe(III) and ADP. The structure activity relationship studies suggest that the introduction of hydroxyl group at 7th and 8th positions in the coumarins, irrespective of Fe(III)–ADP complexation, increases the antioxidative efficacy. No change in uric acid production in the reactions done for all studies further reveals that the coumarin derivatives and their complexes were the only causative factors for O ₂ ^– scavenging and not the suppression of the enzyme, xanthine oxidase.Published online: March 2005     

Synthesis and Antiaggregative Activity of a New RGDF Mimetic

m-[4-Oxo-4-(piperazin-1-yl)butyrylamino)benzoyl-D,L--(3,4-methylenedioxyphenyl)--alanine, a new mimetic of the peptide RGDF, was synthesized. This compound inhibited ADP-induced platelet aggregation in human blood plasma enriched with platelets with IC₅₀ = 3.5 × 10^–8 M.     

Novel synthetic RGD analogs incorporating salicylic acid derivatives show antiplatelet activity in vitro

Summary Continuing our investigational efforts for more active GpIIb/IIIa inhibitors we have synthesized four novel RGD (Arg-Gly-Asp) analogs incorporating salicylic acid derivatives at their N-terminal amino group using the solid phase synthesis. The synthesized compounds 5-Cl-2-HO−C₆H₃−CO−Arg-Gly-Asp(OBzl)-NH₂ and 5-Br-2-HO−C₆H₃−CO-Arg-Gly-Asp(OBzl)-NH₂ were tested for their inhibitory activity on human platelet aggregationin vitro and found to be potent inhibitors of the platelet aggregation with 75 and 67% inhibitory activity respectively. In order to confirm our results flow cytometry with monoclonal antibodies against GpIb, GpIIb/IIIa, GpIIIa and GMP140 receptors was used.     

Action mechanism of the platelet aggregation inducer and inhibitor from Echis carinatus snake venom

Platelet aggregation inducer and inhibitor were isolated from Echis carinatus snake venom. The venom inducer caused aggregation of washed rabbit platelets which could be inhibited completely by heparin or hirudin. The venom inducer also inhibit both the reversibility of platelet aggregation induced by ADP and the disaggregating effect of prostaglandin E₁ on the aggregation induced by collagen in the presence of heparin. The venom inhibitor decreased the platelet aggregation induced by collagen, thrombin, ionophore A23187, arachidonate, ADP and platelet-activating factor (PAF) with an IC₅₀ of around 10 μg/ml. It did not inhibit the agglutination of formaldehyde-treated platelets induced by polylysine. In the presence of indomethacin or in ADP-refractory platelets or thrombin-degranulated platelets, the venom inhibitor further inhibited the collagen-induced aggregation. Fibrinogen antagonized competitively the inhibitory action of the venom inhibitor in collagen-induced aggregation. In chymotrypsin-treated platelets, the venom inhibitor abolished the aggregation induced by fibrinogen. It was concluded that the venom inducer caused platelet aggregation indirectly by the conversion of prothrombin to thrombin, while the venom inhibitor inhibited platelet aggregation by interfering with the interaction between fibrinogen and platelets.     

Efficient Inhibition of Collagen-Induced Platelet Activation and Adhesion by LAIR-2, a Soluble Ig-Like Receptor Family Member

LAIR-1 (Leukocyte Associated Ig-like Receptor -1) is a collagen receptor that functions as an inhibitory receptor on immune cells. It has a soluble family member, LAIR-2, that also binds collagen and can interfere with LAIR-1/collagen interactions. Collagen is a main initiator for platelet adhesion and aggregation. Here, we explored the potential of soluble LAIR proteins to inhibit thrombus formation in vitro. LAIR-2/Fc but not LAIR-1/Fc inhibited collagen-induced platelet aggregation. In addition, LAIR-2/Fc also interfered with platelet adhesion to collagen at low shear rate (300 s⁻¹; IC₅₀ = 18 µg/ml) and high shear rate (1500 s⁻¹; IC₅₀ = 30 µg/ml). Additional experiments revealed that LAIR-2/Fc leaves interactions between collagen and α2β1 unaffected, but efficiently prevents binding of collagen to Glycoprotein VI and von Willebrand factor. Thus, LAIR-2/Fc has the capacity to interfere with platelet-collagen interactions mediated by Glycoprotein VI and the VWF/Glycoprotein Ib axis.     

Inhibition of benzo[a]pyrene-induced cytotoxicity and cytochrome P450 1A activity by dietary flavonoids in human liver cell model: structure-activity relationship

Inhibition of benzo[a]pyrene (B[a]P)-induced cytotoxicity and cytochrome p450 1A (CYP 1A) activity by flavonoids (1–100 M) was examined in terms of the structure-activity relationship in the human liver-derived cell model (HepG2). Two hydroxyl groups in the 5- and 7-position of flavonoids were essential to inhibit B[a]P-induced cytotoxicity. Generally, flavones (IC₅₀; 5.0–17.2 M) were more potent than the corresponding flavonols (IC₅₀; 42.7–131.8 M), and flavonoids such as apigenin (IC₅₀; 7.2 M) were more active than the corresponding isoflavonoids, genistein (IC₅₀; 61.7 M). The planar structure of flavone proved to be important in inhibiting B[a]P-induced toxicity and CYP 1A activity. The inhibitory effect of flavonoids on B[a]P-induced CYP 1A activity was correlated well with the inhibition of B[a]P-induced cytotoxicity (r=0.635, p<;0.01).     

Platelet aggregation inhibitory activity of mutant proinsulin with C-peptide replaced by CRGDSC sequence

A CRGDSC peptide was introduced into an inactive human proinsulin molecule between the B30 site and A1 site to replace the C-peptide. The constructed RGD-proinsulin gene was overexpressed in E. coli and the protein purified. It showed an inhibitory activity of platelet aggregation with an IC₅₀ value of 0.35 M, while native insulin and proinsulin as controls did not exhibit any inhibitory activity. Meanwhile, the RGD-proinsulin demonstrated only 0.05% of insulin receptor binding activity and almost no insulin activity in in vivo assay.     

Potentiation of anti-aggregating activity of PGI2 by 7-ethoxycarbonyl-6,8-dimethyl-4-hydroxymethyl-1(2H)-phthalazinone (EG-626) in rabbit platelets in vitro

Anti-aggregating activity of 7-ethoxycarbonyl-6,8-dimethyl-4-hydroxymethyl-1(2H)-phthalazinone (EG-626) was tested using rabbit platelets in vitro. EG-626 alone, when added before, prevented platelet aggregation induced by ADP, as did PGI₂, papaverine and dipyridamole. Spontaneous disaggregation was also accelerated when EG-626 was added after the maximal aggregation induced by ADP. EG-626 alone also inhibited platelet aggregation induced by collagen and arachidonic acid. ID₅₀s of these agents in ADP-induced aggregation were 7–9 nM for PGI₂, 223 μM for EG-626, 266 μM for papaverine and 957 μM for dipyridamole. When EG-626 was used in combination with PGI₂, a threshold dose (50 μM) of EG-626 potentiated the anti-aggregating effect of subthreshold dose (3 nM) of PGI₂ upto 100% inhibition in collagen-induced platelet aggregation. The marked potentiating effect of EG-626 was accompanied by an accumulation of cyclic AMP in the platelets. These effects might be due to inhibition of phosphodiesterase. Papaverine and dipyridamole, other phosphodiesterase inhibitors, also potentiated the anti-aggregating activity of PGI₂. The activity of papaverine, however, was one eighth of EG-626 and that of dipyridamole was much less. The most effective combination of PGI₂ and EG-626 to induce 50% inhibition was obtained with 20% of ID₅₀ of each agent, whereas that of PGI₂ and papaverine or dipyridamole was 39 or 41%, respectively.     

Variability in blood platelet inhibitory activity of Allium (Alliaceae) species accessions

In the past 20 yr, the presence of blood platelet inhibitory activity in plant species from the genus Allium has been confirmed by a range of clinical and in vitro investigations. Although a number of Allium species have been identified as possessing antiplatelet properties, little is known of the variability for this trait among accessions in these species. Experiments were conducted to assess variability in antiplatelet activity of 58 Allium (Alliaceae) accessions. Extracts were prepared from a diverse collection of 16 Allium species accessions, 33 cultivated accessions of Allium cepa including standard cultivars, inbred lines, and open-pollinated populations, and nine Allium cepa plant introductions of diverse origin. Platelet inhibitory activity was determined via a platelet aggregation assay with human platelet-rich plasma. Relative in vitro inhibition of platelet aggregation was measured for each accession and control samples, and inhibition constants (IC₅₀) were calculated. Dose-dependent inhibition was observed and measured for each Allium accession. Significant (P < 0.01) IC₅₀ variability was detected among accessions, with the lowest accession IC₅₀ values exhibiting nearly 50-fold greater inhibition of aggregation than the highest accession IC₅₀ values. IC₅₍₎ variability among Allium cepa accessions was ≈ 12 times less than among Allium species accessions. Results from this investigation demonstrate substantial variability for efficacy of the antiplatelet factor among Allium accessions.     

Hydrolysis of an orally active platelet inhibitory prostanoid amide in the plasma of several species

The prostanoid 3-oxa-4,5,6-trinor-3,7-inter-m-phenylene-PPGE₁-amide (OI-PGE₁-amide) has a prolonged duration of oral platelet aggregation inhibitory activity when compared to the parent free acid (OI-PGE₁) in the rat. When incubated in rat plasma at 1μg/ml for 30 seconds prior to addition of ADP, OI-PGE₁-amide inhibits in vitro rat platelet aggregation ≈50%. OI-PGE₁ inhibits at 1 ng/ml. Inhibition of platelet aggregation by plasma incubated with OI-PGE₁-amide (1μg/ml) increases with time and the rate of this increase differs with species. Incubation of OI-PGE₁ in plasma does not result in an increase of platelet inhibitory activity with time. The increase of platelet inhibitory activity was assumed to indicate hydrolysis of OI-PGE₁-amide to the more active OI-PGE₁. A compound, different from OI-PGE₁-amide, was isolated by an ion exchange/silica gel separation sequence from an incubation of OI-PGE₁-amide in rat plasma. It had potent platelet aggregation inhibitory activity. This material was shown to be OI-PGE₁ by thin-layer chromatography, gas chromatography and mass spectral analysis. Studies with [³H]-OI-PGE₁-amide confirmed the formation of OI-PGE₁ in plasma incubations. Amide hydrolytic activity was significantly different between species, the rank order being: rat > guinea pig > monkey human > dog. This relationship corresponded with that determined by measuring the increase in platelet inhibitory activity with time in plasma incubations of OI-PGE₁-amide reported above. Present data indicate that (a) OI-PGE₁-amide is hydrolyzed to the parent acid by plasma enzymes of several species and (b) hydrolytic activity of plasma varies widely between species.     

Elucidation of the Role of Sugar Chains in Glycosylated-enzymes Using Endo-γ-N-acetylglu-cosaminidase from Flavo-bacterium sp.

In the course of screening for anti-platelet principles produced by micro-organisms, strong anti-platelet activity was detected in the culture broth of Aspergillus fumigatus Fres. The purified active compound was identified as gliotoxin. Gliotoxin inhibited ADP-induced aggregation as well as collagen- or arachidonate-induced aggregation of rabbit platelets (IC₅₀ = about 27 μm) and also accelerated the dissociation of aggregates. Gliotoxin also inhibited the heat hemolysis of rabbit erythrocytes, suggesting that this agent is a membrane-stabilizing anti-aggregant. The disulfide structure in the gliotoxin molecule was responsible for the inhibitory activity, because des- thiogliotoxin had effects on neither platelet aggregation nor heat hemolysis of erythrocytes.